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Open Roads Forum  >  Tech Issues

 > [SOLVED] Can an inverter on a car charge the house battery?

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pianotuna

Regina, SK, Canada

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Posted: 01/04/21 02:17am Link  |  Quote  |  Print  |  Notify Moderator

SlowBro,

The inverter is too small IF the converter will go to bulk charging mode.

The WF-8945 draws 780 watts or 11.6 amps @ 120 volts, when running flat out.

11.6 amps @ 120 volts = 116 amps @ 12 volts.

Sizing the inverter suggests that 1000 watts might be a better fit.

C/8 is the acceptance rate of a lead acid battery at 85% state of charge. If it is at 50% that may be a LOT higher.

Most alternators have a duty cycle of 2/3, so if the battery bank is "hungry" the alternator may let out the magic blue smoke, with the caveat that the cables to the inverter are thick enough to handle that much current.

On the other hand, WF converters are FAMOUS for not going into bulk charging mode. In that case the 800 watt inverter would not over load, nor would the alternator fail.

It is far better and probably cheaper to use a dc to DC converter between the alternator and the battery bank. That way charge current may be set where you want it.

Do a short time test, preferably with the house batteries cycled to 50%.

If the voltage drops to 12.7 on the engine battery STOP the test.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, soon to have SiO2 batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

2112

Texas

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Posted: 01/04/21 03:14am Link  |  Quote  |  Print  |  Notify Moderator

Can you get your vehicle close enough to the house battery to use jumper cables? That's how I do it.

Most newer model vehicles are smart enough to not over-tax the alternator.


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valhalla360

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Posted: 01/04/21 05:46am Link  |  Quote  |  Print  |  Notify Moderator

Is this a high output 150amp alternator or the stock alternator?

Most stock OEM alternators are designed to put out big amperage for a few minutes right after start up then drop back to a much lower output.

Also, were is the inverter? You mention in the "cabin" but does that mean in the vehicle or in the trailer cabin?

Be prepared with a jump pack if you try this (or at a site with shorepower) as you could find your starter battery dead as the inverter drags it down.

Once you get past the inverter of appropriate size, no issues charging...at that point it's no different than being plugged into shore power.


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Gdetrailer

PA

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Posted: 01/04/21 06:36am Link  |  Quote  |  Print  |  Notify Moderator

SlowBro wrote:

Inverter is a WF-8945. Charge profile is on page 7 of this PDF. Charge current isn’t listed, but the circuit branch is I believe only 30A, which the inverter can handle just fine.
The PDF references another document for more details on the charge profile, that I’ll have to check out tomorrow.

https://wfcoelectronics.com/wp-content/uploads/2020/11/8900-Series-Manual-Updated.pdf

Sitting in a car idling with the A/C on, radio going, lights on, uses about the same amount of current the inverter would use, and those kinds of conditions are seen regularly. I believe I should be fine.

https://www.aa1car.com/library/electrical_amp_loads.htm


[emoticon]

I know that this is/has been done by quite a few "members" of this forum, BUT, do you realize that this is the same thing as using a "cannon" to swat a fly?

745W = 1 HP..

Your gonna "idle" a large vehicle engine for hrs on end to do something that can be done with a very small portable gen..

One should also realize that the alternator on said vehicle is at MINIMAL charging when engine is at idle.. The alternator should be operated ABOVE idle speeds when drawing a lot of current (as in attempting to charge via a inverter), otherwise you do risk running your starting battery down in the process.

Most car alternators are designed for INTERMITTENT heavy charging, they are compact, have the voltage regulator and the diodes built in, that package gets pretty darn hot under normal use, heat buildup is the enemy..

Getting back to the 1 HP.. depending on size of your engine, you are most likely going to use more fuel than you would with a small purpose built inverter gen. Small gens sip fuel, less than a PINT per hr, larger vehicle engines can consume GALLONS per hr at idle

Your gonna put a lot of idle time on a very expensive vehicle engine and transmission which if it breaks down is going to leave you somewhere that you don't want to be and it will cost you many times more to repair than throwing away a small gen.. Excessive idling also can be very hard on the emissions systems and if a Diesel, something called "wet stacking" can happen and be harmful and shorten the life of the engine.

I personally would never intentionally do this to any of my $50K trucks, too expensive to replace a truck, too expensive to replace/repair engine and transmission (a short block can cost as much as $10K just for the block and transmissions can cost $5K not including labor).. Too expensive and difficult to troubleshoot modern vehicle computerized systems (most mechanics charge at a min of $150 per hr now days)..

Just because it has been or is being done, doesn't mean that it is a good idea to do it.

Buy a small gen and or solar, instead of cobbling up some half baked idea.

cavie

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Posted: 01/04/21 07:04am Link  |  Quote  |  Print  |  Notify Moderator

here we go. Reinventing the wheel again!!


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SlowBro

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Posted: 01/04/21 07:12am Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer wrote:

I know that this is/has been done by quite a few "members" of this forum


Oh sorry, I searched before asking and couldn't find others doing it. Maybe my search skills are off.


Gdetrailer wrote:

Your gonna "idle" a large vehicle engine for hrs on end to do something that can be done with a very small portable gen..


I do have one of those cheap and inefficient Chinese generators and my van uses less gas per hour idling, despite having a larger engine.


Gdetrailer wrote:

Most car alternators are designed for INTERMITTENT heavy charging


Oh then that's perfect. I have in mind intermittent use, otherwise I would be buying solar.


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SlowBro

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Posted: 01/04/21 07:18am Link  |  Quote  |  Print  |  Notify Moderator

pianotuna wrote:

The inverter is too small IF the converter will go to bulk charging mode.

The WF-8945 draws 780 watts or 11.6 amps @ 120 volts, when running flat out.


780 watts at 80% efficiency and 14.4V = 43.3A which is in line with the rating on the plate.

Wouldn't that be for everything though? Lights, accessories, and battery? The branch for the battery is 30A so I can't see how it'd be more than that. Unless the branch circuit isn't where the battery is plugged in... I'm not near the RV at the moment to check.


pianotuna wrote:

Do a short time test, preferably with the house batteries cycled to 50%.

If the voltage drops to 12.7 on the engine battery STOP the test.


Good to know.

SpeakEasy

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Posted: 01/04/21 07:21am Link  |  Quote  |  Print  |  Notify Moderator

It's been mentioned, once, in this discussion, but it bears repeating: you have to connect your inverter directly to your car battery. Do NOT under any circumstances, connect the inverter through your cigarette lighter port.

You probably already know this, but it is important enough to repeat.


-Speak


It's just Mrs. SpeakEasy and me now (empty-nesters). But we can choose from among 7 grandchildren to drag along with us!



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SlowBro

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Posted: 01/04/21 07:23am Link  |  Quote  |  Print  |  Notify Moderator

valhalla360 wrote:

Is this a high output 150amp alternator or the stock alternator?


Sienna stock alternator is rated at 150A.


valhalla360 wrote:

Also, were is the inverter? You mention in the "cabin" but does that mean in the vehicle or in the trailer cabin?


Vehicle cabin.


valhalla360 wrote:

Be prepared with a jump pack if you try this (or at a site with shorepower) as you could find your starter battery dead as the inverter drags it down.


Good to know. I was told above to watch that voltage stays above 12.7.


valhalla360 wrote:

Once you get past the inverter of appropriate size, no issues charging...at that point it's no different than being plugged into shore power.


Yeah, that's what I thought. Was criticized above for "reinventing the wheel" when I thought, surely someone somewhere has done this already and I just missed it in my search.

SlowBro

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Posted: 01/04/21 07:24am Link  |  Quote  |  Print  |  Notify Moderator

SpeakEasy wrote:

Do NOT under any circumstances, connect the inverter through your cigarette lighter port.


Yes indeedie. The inverter has ring terminals for the battery.

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